In mobile communication systems, wireless terminals are connected to a mobile communication network via one or more base stations, for example to receive and make phone calls. The wireless terminal and the mobile communication network exchange messages also at other times than during the active connection, for instance to select the most suitable base station for the communication between the wireless terminal and the mobile communication network. A base station via which the wireless terminal communicates with the mobile communication network is generally called a serving base station. Typically, the serving base station is such a base station which is located closest to the wireless terminal, but in some conditions it is possible to attain a better quality for the connection via another base station. Such a situation may occur for example in blind spots, for example as a result of signal attenuation caused by buildings and/or geography. Furthermore, when the wireless terminal moves in the borderline of the range of the base station or outside the same, the quality of the connection may be weakened to such an extent that attempts are made to change the serving base station to another base station of the mobile communication network.
To estimate the quality of connection, measurements are performed in the wireless terminal with the aim of finding out the strength of the signal transmitted by one or more base stations in the vicinity of the wireless terminal, and possibly also the noise level. The wireless terminal can transmit this measurement data to the mobile communication network in which measurement information is used for selecting the most suitable base station for the connection, and the serving base station is changed, if necessary.
In the GSM system a time division/frequency division multiple access system (TDMA/FDMA) according to the GSM system is used in the physical layer (radio channel). The basic transmission unit is called a burst, which is formed of a predetermined number of bits that are transmitted to the radio channel. Eight bursts, i.e. time slots constitute one TDMA frame. These frames are utilized for forming a wider multiframe that comprises 51 TDMA frames. These multiframes are used for the implementation of logical channels. The multiframe contains so-called frequency control channels (FCCH) and synchronizing channels SCH. In the frequency control channels a frequency correction burst (FCB) is transmitted at intervals, by means of which the receiver of the wireless terminal can conduct fine-tuning of the frequency of its frequency synthesizer. In the frequency correction burst a pure sine wave (PSW) (i.e. a sort of a carrier wave) is transmitted, the frequency of which is 67.71 kHz. In the synchronizing channel a synchronization burst SB is transmitted at intervals, by means of which the receiver can conduct the synchronization with the multiframe. The frequency correction burst and the synchronization burst are transmitted in successive frames in the same time slot, wherein the time between these bursts is eight time slots (=1 frame). FIG. 1 illustrates this multiframe structure in a reduced manner. Furthermore, the multiframe of the GSM system also comprises other logical channels, such as a broadcast control channel BCCH. Other channels have also been determined, but it is not necessary to discuss them in more detail in this description. In other mobile communication systems corresponding frame structures and logical channels have also been determined.
When the wireless terminal is searching for radio channels for example in a situation where the wireless terminal is not logged into the network or the connection to the serving base station has been terminated, the wireless terminal searches for the sine wave transmitted by the base station. This sine wave is for example a synchronizing signal preceding the SCH frame. The receiver of the wireless terminal provides samples of the antenna signal (Rx-sample) at intervals of approximately 30 microseconds. A measured value is calculated from these samples, which measured value can be utilized for determining whether the sine wave in question can be found in the channel under examination. The duration of one measurement in a wireless terminal of prior art falls in the order of 2 μs, depending on the computing efficiency of the digital signal processing unit. This time is used by the digital signal processing unit of the wireless terminal to calculate the measured value, during which time the digital signal processing unit is, in principle, incapable of performing other actions. In practice, the digital signal processing unit is in a wait mode also during the time between the measurements, because this time is not, in practice, sufficient for performing demanding signal processing measures. This complicates for example the act of performing other demanding signal processing measures in wireless terminals.
Thus, during the process of searching for the base station signal, wireless terminals of prior art are incapable of performing e.g. functions related to a voice controlled user interface, such as speech recognition, speech recording, playback of recordings, changing of the user profile, etc., because the short waiting periods between the calculations are too short for executing voice control applications. The search for the base station signal is performed for example always when the wireless terminal searches for the mobile communication network. If the wireless terminal is in such a location in which the reception from the base stations of the mobile communication network is poor, the wireless terminal may search for the base station signal frequently. This means that to be able to use said voice controlled user interface, the wireless terminal must be logged into the mobile communication network. However, situations may occur in which the user of the wireless terminal would like to perform functions of the voice controlled user interface irrespective of the fact whether or not the wireless terminal is logged into the mobile communication network. It should, for example, be possible to listen to recordings also when the wireless terminal is not logged into the mobile communication network.
The wireless terminal can also search for the base station signals when it is logged into the mobile communication network. Thus, the adjacent cells are monitored, in case it is necessary to change the serving base station. Thus, on the basis of these measurements of the adjacent cells it is possible to make conclusions which base station is selected as a new serving base station in the mobile communication network.
There are different audio applications under development for wireless terminals, such as the above-mentioned voice-controlled user interface. This requires that voice recognition measures are conducted for example for recognition of voice commands. In some wireless terminals that are under development, the intention is to implement recording and listening functions of recordings and other corresponding operations. Such voice recognition applications require a great deal of real-time processing capacity. Especially in conventional portable wireless terminals there is, however, only one digital signal processing unit available, which is used in many signal processing functions. One solution for this limited processing capacity is to prevent the search for base station signals while audio applications are executed. The execution of audio application should not, however, last longer than a few seconds, because there is a risk that the wireless terminal loses the connection to the mobile communication network. Said time limit may be sufficient for recognition of short voice commands, but often this period of time is not sufficient for example for storing and listening to recordings.
Hereinabove, audio applications are presented only as an example, but corresponding problems may also occur in other applications in which a great amount of real-time processing is required during measurements of the base station signal.